Tom Mackay reckons his backyard in Duluth shows what is happening in Lake Superior as well as any place. In November 2005, the metre-tall wooden “Bay Ness Monster” statue he installed in the water just off his home dock was submerged up to its gaping mouth. Today, his would-be water serpent is high and dry.

For residents of this lakeside Minnesota city, located more than 3000 kilometres by boat from the open Atlantic, the transformation is disturbing. Lake Superior, the largest body of fresh water in the world by surface area, is experiencing its lowest water levels since the record set in 1926. The lake is down by 34 centimetres from a year ago, and more than half a metre below its long-term mean. At least part of the drop can be attributed to a multi-year drought that has been particularly severe since 2006. More troubling, however, is evidence that global warming is driving a long-term shrinkage of this massive natural reservoir.

A rapidly warming lake is the key to understanding the change, says Jay Austin, a limnologist at the University of Minnesota Duluth’s Large Lakes Observatory. Earlier this year he reported that Superior’s surface waters had warmed by about 2.5 °C since 1979 – far more than average air temperatures in the region during the same period (Geophysical Research Letters, vol 34, p L06604). Austin’s findings link the warming to a reduction in winter ice cover on the lake. The less ice is present to reflect sunlight, the more solar energy the lake can absorb. On average, the onset of summer warming of the lake is happening half a day earlier each year. The reduced ice cover also contributes to shrinkage by allowing more evaporation. “Most of the evaporation goes on in winter,” Austin says, as cold, dry air swoops over the warmer lake. Without the ice cap to block evaporation, water losses increase.

Cynthia Sellinger of the National Oceanic and Atmospheric Administration’s Great Lakes Environmental Research Laboratory in Ann Arbor, Michigan, agrees. While the lake’s level has dropped precipitously since last year, Sellinger has tracked a longer-term decline of an average of 10 millimetres per year since 1978. Evaporation has increased by an average of 4.6 millimetres per year over the same period, she says, while precipitation has decreased by 4.1 millimetres per year. These drops are consistent with climate change models, Austin says, which predict a decline in Great Lakes levels of 0.5 to 2.5 metres with a doubling of atmospheric carbon dioxide.

The falling water level is already affecting Lake Superior’s shipping industry. Freighters carry less cargo now for fear of running aground. Natural beds of wild rice growing in the lake’s shoreside wetlands and harvested by Native Americans are also threatened. The long-term effects of prolonged warming on Lake Superior’s aquatic ecosystems are not yet known.

Austin has found similar increases in temperature and length of warming season in lakes Michigan and Huron, which are both at lower than average levels. In contrast, lakes Erie and Ontario have captured plenty of rain in recent years, Sellinger says, including the remnants of hurricanes Katrina and Dennis, and are at above average levels.

Last month the US-Canadian International Joint Commission, which manages waters on the boundary, commissioned a study of water management of the upper Great Lakes to determine whether managing lake outflow differently could improve levels.

Meanwhile, Mackay’s friends have had to find a deeper place than his dock to moor their boats for the season. “If we get some rain, hopefully we’re back in business,” Mackay says. Austin is less optimistic. This is the season when the lake should be rising quickly, and it’s not keeping pace, he says. “2007 is shaping up to be a very low year.

From issue 2606 of New Scientist magazine, 30 May 2007, page 8-9